Geometry Nodes - How to assign vertex group in geometry nodes?
If I’m interpreting your question correctly, and you just want to restrict the set of points where instances are created using a vertex group, you can use the Point Separate node:

In this example, my vertex group is named “no cubes”.
If you want the inverse, i.e. you want the cubes to only be instanced at the vertices contained in the vertex group, then you can use the Geometry 2 output of the Point Separate node, instead.